When we look up at the night sky, it's easy to appreciate how scientists have gained insights from the apparent movements of those twinkling orbs. But it's striking how much of astronomy involves looking for indirect clues to something unseen. As an undergraduate, I reflected on how Neptune revealed itself by subtly shifting the orbit of Uranus. Then Pluto was found when astronomers thought (incorrectly) that something was pulling on Uranus and Neptune.

Now, as staff editor Michael D. Lemonick writes, “Something very odd seems to be going on out beyond Pluto.” Could it be another hidden world? In our cover story, “The Search for Planet X,” Lemonick describes how “super Earths”—planets with up to roughly 10 times Earth's mass—could be circling the sun beyond our current range of discovery. Today they are too far to be detected, but perhaps future observatories could confirm them, if they exist. Meanwhile we can enjoy the tale of the hunt for the latest icy quarry in the cloud of objects beyond Neptune known as the Kuiper belt. Turn to page 30. And if you enjoy quests for objects in the night sky, after that you might jet to “My Life as a Comet Hunter,” where David H. Levy describes a “cosmic passion” that's lasted for the past 50 years.

In contrast to the expanding universe, Robert Engelman writes, “Earth is a finite place.” It's increasingly clear that our species must learn how to live within our means—whether it comes to energy, food, water or any other resource. One way we do that is to use less as a species. And fewer people use fewer resources.

We have been improving on that score, with a global average of 2.5 births per woman, about half the level of six decades ago. But challenges remain where populations are still rising steeply. In “Six Billion in Africa,” Engelman reports that fertility remains high in most of Africa's 54 countries—high enough that by 2100 the continent's 1.2 billion could swell to between three billion and 6.1 billion, adding further challenges to the economies and systems in some of the world's poorest regions. Giving women opportunities and choices over their lives has perhaps the greatest potential. As Engelman says, “The empowerment of women needs no demographic justification,” but it is nonetheless a critical factor in helping Africa move toward a more prosperous future.